New windows based on traditional designs are fun to do and always turn out well. Often we can copy and/or adapt traditional designs and there are plenty of examples in books or on the web. have a look around and choose one that attracts you, bearing in mind that everything can be adjusted to suit your tastes.
Sometimes it is not possible to obtain old stained glass, but there are plenty of new types from which to choose.
If you want an old window to be rebuilt, perhaps due to the lead having failed, always take great care when removing it to avoid damaging the glass, which can form the basis of the new, replacement window.